1. Field of the Invention
The present invention relates to an acceleration sensor used in, for example, an air bag system of an automobile for detecting an acceleration caused by, for example, impact.
2. Description of the Related Art
A conventional acceleration sensor 50 will be described with reference to FIGS. 4A, 4B, 5A and 5B. FIG. 4A is a top view of an acceleration sensor 50, and FIG. 4B is a cross sectional view of the acceleration sensor 50 along lines 4B--4B in FIG. 4A.
The acceleration sensor 50 includes a base 21, a circuit board 23 bonded on a surface of the base 21, a detection section 24 bonded on a surface of the circuit board 23 for detecting an acceleration to be sensed, and a cover 22 fixed to the base 21 by welding or the like for creating a sealed inner space 27 in which the circuit board 23 and the detection section 24 are located. The cover 22 prevents foreign substances from entering the inner space 27. The detection section 24 on the circuit board 23 is connected to terminals 25 fixed to the base 21 in the state of being electrically insulated from the base 21, and is connected to an external circuit via the terminals 25. The base 21 has holes 21a in the vicinity of two ends thereof for attaching the acceleration sensor 50 to a prescribed position of an object for which an acceleration is to be sensed, for example, an automobile.
The acceleration sensor 50 is used in an electric circuit for controlling the operation of, for example, an air bag system of an automobile. The acceleration sensor 50 is attached to a prescribed position of an automobile by inserting screws through the holes 21a, and the terminals 25 are electrically connected to a circuit board (not shown) on which the electric circuit for controlling the operation of the air bag system is located.
FIG. 5A is a top view of the detection section 24, and FIG. 5B is a cross sectional view of the detection section 24 along lines 5B--5B in FIG. 5A.
The detection section 24 includes a fixed portion 24b and a movable cantilever beam 24a extended from the fixed portion 24b. The detection section 24 includes a space 24e below the cantilever beam 24a for allowing the cantilever beam 24a to move. The cantilever beam 24a includes a base portion 24c and a weight portion 24d extending out from the base portion 24c. The weight portion 24d is thicker than the base portion 24c. A strain detector 26 is located on the base portion 24c of the cantilever beam 24a.
The conventional acceleration sensor 50 operates in the following manner.
The acceleration sensor 50 is attached to, for example, an automobile by screws or the like. Movement of the automobile is transmitted to the base 21. Then, the movement is transmitted to the fixed portion 24b via the circuit board 23, and further to the cantilever beam 24a. Thus, the cantilever beam 24a is moved by the force of inertia of the weight portion 24d. By such movement of the cantilever beam 24a, the base portion 24c is bent in proportion to the acceleration caused by the movement. By appropriately setting the thickness of the base portion 24c and the weight portion 24d, the amount of bending is detected as a change in the resistance of the strain detector 26. A signal indicating the amount of bending detected by the strain detector 26 is amplified by an amplifier (not shown) provided on the circuit board 23 and is output from the terminals 25 to the circuit board in the automobile as a signal indicating the amount of detected acceleration.
In the conventional acceleration sensor 50, the amplifier and the detection section 24 are provided on the circuit board 23. Thus, the entire structure of the acceleration sensor 50 is enlarged, which enlarges the air bag system provided with the acceleration sensor 50. Moreover, since the conventional acceleration sensor 50 is attached to the object for which an acceleration is to be sensed by inserting screws through the holes 21a, the liberty in attachment is restricted and the attachment process is complicated.